Blot scallops thoroughly with paper towels and remove the small side muscle. Season all over with salt and pepper.
Heat a large stainless steel or cast-iron skillet over medium-high. Swirl in the olive oil until shimmering.
Sear scallops in a single layer without crowding, 1½–2 minutes, until a deep golden crust forms. Do not move them.
Flip and cook 1–1½ minutes more, just until opaque. Transfer to a plate and tent loosely with foil.
Reduce heat to medium. Add butter to the skillet to melt, then stir in garlic and cook 30 seconds until fragrant.
Pour in wine or broth and scrape up browned bits. Simmer about 1 minute to slightly reduce.
Stir in lemon juice and zest. Taste and adjust seasoning with a pinch of salt if needed.
Return scallops and any juices to the pan. Spoon sauce over and warm 30–60 seconds. Finish with parsley and red pepper flakes.
Serve right away with extra lemon if you like.